Method for fabricating an electrically heatable coated transparency

ABSTRACT

A method for fabricating an electrically heatable coated transparency, particularly useful as a vehicle windshield, is disclosed. The method involves sputtering on a substrate surface layers of metal oxide, metal-containing primer, infrared reflective metal, metal-containing primer and metal oxide to produce an electroconductive coating, and applying first and second bus bars to the substrate in contact with the coating.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of Ser. No. 07/333,069 filed Apr. 3,1989, now U.S. Pat. No. 5,028,759; which is a continuation-in-part ofSer. No. 176,511 filed Apr. 1, 1988, now U.S. Pat. No. 4,898,790; whichis a continuation-in-part of Ser. No. 947,799 filed Dec. 29, 1986, nowU.S. Pat. No. 4,806,220.

BACKGROUND OF THE INVENTION

This invention relates to a low emissivity film in an electricallyheated transparency such as may be employed in a vehicle to providedefrosting, deicing, or defogging capability.

It is known to pass electric current through a transparent conductivecoating on a transparency in order to raise the temperature of thetransparency. Generally, a source of electrical potential is connectedto the conductive coating by way of a pair of bus bars along oppositesides of the area of the transparency to be heated. The bus bars havelow resistivity relative to the coating and are intended to distributethe current evently over the area to be heated. The bus bars may becomprised of metallic foil strips, but in the case of glasstransparencies they preferably are comprised of a metallic-ceramic fritmaterial fused onto a surface of the transparency.

A typical arrangement includes bus bars configured as substantiallyparallel stripes on opposite sides of the heated area, with electricalleads attached to each bus bar and extending away from the oppositeedges of the transparency as shown in U.S. Pat. Nos. 4,323,726 to Crisset al and 4,668,270 to Ramus. Locating the leads on the same side of thetransparency and preferably closely adjacent to each other isadvantageous for the sake of easier installation of the transparency inthe vehicle and simplifying the connection with the electrical powersource as shown in U.S. Pat. Nos. 3,895,213 to Levin and 4,543,466 toRamus.

A preferred bus bar arrangement involves connection of the remote busbar to the electrical circuit by way of two conductive extensions of thebus bar, each extending from opposite ends of the remote bus bar alongopposite ends of the transparency as described in U.S. Ser. No. 138,008filed Dec. 28, 1987. The conductive extensions are insulated from theconductive coating on the transparency, preferably by omitting ordeleting the coating in the marginal area near the extensions.

U.S. Pat. No. 4,094,763 to Gillery et al discloses producingtransparent, electroconductive articles by cathode sputtering metalssuch as tin and indium onto refractory substrates such as glass at atemperature above 400° F. in a low pressure atmosphere containing acontrolled amount of oxygen.

U.S. Pat. No. 4,113,599 to Gillery teaches a cathode sputteringtechnique for the reactive deposition of conductive indium oxide inwhich the flow rate of oxygen is adjusted to maintain a constantdischarge current while the flow rate of argon is adjusted to maintain aconstant pressure in the sputtering chamber.

U.S. Pat. No. 4,462,883 to Hart discloses a low emissivity coatingproduced by cathode sputtering a layer of silver, a small amount ofmetal other than silver, and an antireflection layer of metal oxide ontoa transparent substrate such as glass. The antireflection layer may betin oxide, titanium oxide, zinc oxide, indium oxide, bismuth oxide orzirconium oxide.

High transmittance, low emissivity coatings as described above generallycomprise a thin metallic layer, for infrared reflectance and lowemissivity, sandwiched between dielectric layers of metal oxides toreduce the visible reflectance. These multiple layer films are typicallyproduced by cathode sputtering, especially magnetron sputtering. Themetallic layer may be gold or copper, but is generally silver. The metaloxide layers described in the prior art include tin oxide, indium oxide,titanium oxide, bismuth oxide, zinc oxide, zirconium oxide and leadoxide. In some cases, these oxides incorporate small amounts of othermetals, such as manganese in bismuth oxide, indium in tin oxide and viceverse, to overcome certain disadvantages such as poor durability ormarginal emissivity. However, all of these metal oxides have somedeficiency.

U.S. Pat. No. 4,610,771 to Gillery, the disclosure of which isincorporated herein by reference, provides a novel film composition ofan oxide of a zinc-tin alloy, as well as a novel multiple-layer film ofsilver and zinc-tin alloy oxide layers for use as a high transmittance,low emissivity coating.

While multiple-layer, low-emissivity, high transmittance films have beenmade sufficiently durable for architectural applications in multipleglazed window units, such films were not sufficientlytemperature-resistant to withstand high temperature processing, such asbending, tempering or laminating. Moreover, it is desirable to have acoating which serves both as a low emissivity film to reduce heat gainin an enclosed space and as a current-carrying heatable film fordefogging, defrosting or deicing a transparency, particularly in avehicle.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is an exploded, cross-sectional, enlarged view of a typicallaminated windshield construction of the present invention.

FIG. 2 is a schematic, elevational view of a windshield incorporating apreferred bus bar arrangement.

FIG. 3 is a cross-sectional, enlarged view, not to scale, of the coatingin FIGS. 1 and 2.

SUMMARY OF THE INVENTION

The present invention involves a multiple-layer coating which issufficiently temperature resistant to enable coated substrates such asglass to be subjected to high temperature processing such as bending,annealing, tempering, laminating or glass welding, and to enableconnection to a power source for heating. The multiple-layer coating ofthe present invention comprises a first antireflective metal oxide layersuch as an oxide of zinc and tin, an infrared reflective metal layersuch as silver, a primer layer of titanium metal and titanium oxide, asecond antireflective metal oxide layer and, preferably, an exteriorprotective layer of titanium metal or titanium oxide.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A film composition preferably comprising an oxide of a metal or metalalloy is preferably deposited by cathode sputtering, preferablymagnetron sputtering. A cathode target is prepared comprising thedesired metal or metal alloy elements. The target is then sputtered in areactive atmosphere, preferably containing oxygen in order to deposit ametal or metal alloy oxide film on a surface of a substrate.

A preferred metal alloy oxide in accordance with the present inventionis an oxide of an alloy comprising zinc and tin. A zinc/tin alloy oxidefilm may be deposited in accordance with the present invention bycathode sputtering, preferably magnetically enhanced. Cathode sputteringis also a preferred method for depositing high transmittance, lowemissivity films in accordance with the present invention. Such filmstypically comprise multiple layers, preferably a layer of a highlyreflective metal such as gold or silver sandwiched betweenantireflective metal oxide layers such as indium oxide or titaniumoxide, or preferably an oxide of an alloy of zinc and tin whichpreferably comprises zinc stannate.

While various metal alloys may be sputtered to form metal alloy oxidefilms, in order to produce a preferred high transmittance, lowemissivity multiple layer film in accordance with the present invention,alloys of tin and zinc are preferred. A particularly preferred alloycomprises zinc and tin, preferably in proportions of 10 to 90 percentzinc and 90 to 10 percent tin. A preferred zinc/tin alloy ranges from 30to 60 percent zinc, preferably having a zinc/tin ratio from 40:60 to60:40. A most preferred range is 46:54 to 50:50 by weight tin to zinc. Acathode of zinc/tin alloy reactively sputtered in an oxidizingatmosphere results in the deposition of a metal oxide layer comprisingzinc, tin and oxygen, preferably comprising zinc stannate, Zn₂ SnO₄.Such a metal alloy oxide layer may also be deposited by simultaneouslysputtering zinc and tin in suitable proportions. However, it is moreconvenient to use a metal alloy target.

In a conventional magnetron sputtering process, a substrate is placedwithin a coating chamber in facing relation with a cathode having atarget surface of the material to be sputtered. Preferred substrates inaccordance with the present invention include glass, ceramics andplastics which are not detrimentally affected by the operatingconditions of the coating process.

The cathode may be of any conventional design, preferably an elongatedrectangular design, connected with a source of electrical potential, andpreferably employed in combination with a magnetic field to enhance thesputtering process. At least one cathode target surface comprises ametal alloy such as zinc/tin which is sputtered in a reactive atmosphereto form a metal alloy oxide film. The anode is preferably asymmetrically designed and positioned assembly as taught in U.S. Pat.No. 4,478,702 to Gillery et al, the disclosure of which is incorporatedherein by reference.

In a preferred embodiment of the present invention, a multiple layerfilm is deposited by cathode sputtering to form a high transmittance,low emissivity coating. In addition to the metal alloy target, at leastone other cathode target surface comprises a metal to be sputtered toform a reflective metallic layer. At least one additional cathode targetsurface comprises the titanium to be sputtered to deposit a titaniumoxide layer. A durable multiple layer coating having a reflectivemetallic film in combination with an antireflective metal alloy oxidefilm is produced as follows, using a titanium oxide primer layer toimprove the adhesion between the metal and metal oxide films, whichprimer layer also provides high-temperature resistance to themultiple-layer coating in accordance with the present invention so thatthe resultant coated article may be subjected to high temperatureprocessing, such as bending, annealing, tempering, laminating, or glasswelding without deterioration of the coating.

While primer layers of the prior art are preferably of minimalthickness, the thickness of the primer layer of the present invention ispreferably between 10 and 100 Angstroms, most preferably 12 to 30Angstroms. If a single primer layer is deposited over the reflectivemetal film, the thickness is preferably between 20 and 100 Angstroms. Ifthe thickness of the primer layer over the reflective metal layer isless than 20 Angstroms, preferably an additional primer layer at least10 Angstroms thick is deposited between the first antireflective metaloxide layer and the reflective metal layer, and the total thickness ofthe two layers is preferably not more than 100 Angstroms. Although theprimer of the present invention may be deposited as titanium metal, itis necessary for the primer layer to be essentially completely oxidizedto titanium oxide in the final product. While such oxidation can bereadily accomplished by heating for primer layers less than about 50Angstroms thick, the present invention provides for thicker primerlayers wherein the presence of titanium oxide enhances the oxidation ofmetallic titanium allowing thicker primer layers to be employed withoutcompromising optical properties in the multilayer coated product. For aprimer layer under silver, for example, it is preferred to deposittitanium oxide first and then titanium, while for a primer on top ofsilver it is preferred to deposit titanium first and then titaniumoxide. The presence of titanium oxide is more critical in a primer layerunder silver, but in either location, it is preferred to have titaniummetal in contact with silver in the film as deposited. Upon heating,such as during bending or lamination, the titanium oxidizes to titaniumoxide and the transmittance increases.

A multiple-layer low emissivity film is formed on a glass substrate asfollows. A clean glass substrate is placed in a coating chamber which isevacuated, preferably to less than 10⁻⁴ torr, more preferably less than2×10⁻⁵ torr. A selected atmosphere of inert and reactive gases,preferably argon and oxygen, is established in the chamber to a pressurebetween about 5×10⁻⁴ and 10⁻² torr. A cathode having a target surface ofzinc/tin metal is operated over the surface of the substrate to becoated. The target metal is sputtered, reacting with the atmosphere inthe chamber to deposit a zinc/tin alloy oxide coating layer on the glasssurface.

After the initial layer of zinc/tin alloy oxide is deposited$ thecoating chamber is evacuated, and an inert atmosphere such as pure argonis established at a pressure between about 5×10⁻⁴ and 10⁻² torr.Preferably, a cathode having a target surface of titanium is firstsputtered in an oxidizing atmosphere to deposit titanium oxide and thensputtered in an inert atmosphere to deposit titanium in the primer layeras well. A cathode having a target surface of silver is then sputteredto deposit a reflective layer of metallic silver over thetitanium-containing primer layer. A second titanium-containing primerlayer is preferably deposited by sputtering titanium over the reflectivesilver layer. Preferably, the titanium is sputtered in an inertatmosphere to deposit a titanium metal primer layer. Finally, a secondlayer of zinc/tin alloy oxide is deposited over the second titaniumprimer layer under essentially the same conditions used to deposit thefirst zinc/tin alloy oxide layer. The second primer layer may comprisetitanium oxide as well as titanium metal, so long as titanium issputtered in an inert atmosphere over the silver first.

In most preferred embodiments of the present invention, a protectiveovercoat is deposited over the final metal oxide film, The protectiveovercoat is preferably deposited by sputtering over the metal oxide filma layer of a metal such as disclosed in U.S. Pat. No. 4,594,137 toGillery et al. Preferred metals for the protective overcoat includealloys of iron or nickel, such as stainless steel or Inconel. Titaniumis a most preferred overcoat because of its high transmittance. In analternative embodiment, the protective layer may be a particularlychemical resistant material such as titanium oxide as disclosed in U.S.Pat. No. 4,716,086 to Gallery et al, the disclosure of which isincorporated herein by reference.

The chemical resistance of a multiple layer film is most improved bydepositing a protective coating comprising titanium oxide over themultiple layer film. Preferably, the titanium oxide protective coatingis deposited by cathode sputtering at a relatively high deposition rateand low pressure, preferably about 3 millitorr. A protective coatingcomprising titanium oxide may be formed by sputtering titanium in anoxygen-sufficient atmosphere to deposit titanium oxide directly. In analternative embodiment of the present invention, a protective coatingcomprising titanium oxide may be formed by sputtering titanium in aninert atmosphere to deposit a titanium-containing film whichsubsequently oxidizes to titanium oxide -upon exposure to an oxidizingatmosphere such as air.

The coated glass is then preferably fabricated to form a laminatedtransparency comprised of two plies of glass bonded together by aninterlayer of plastic, since that is the typical windshieldconstruction. However, it should be understood that the inventionapplies as well to heated transparencies involving a single ply of glasswith a single ply of plastic, all plastic laminations, and othercombinations. The laminate need not be used as an automobile windshield,but may be a back or side window, sunroof or any transparency for anyvehicle, including aircraft, or any other enclosed space.

As shown in FIG. 1, the transparency is preferably comprised of anoutboard glass sheet 10, a plastic interlayer 11 which may bepolyvinylbutyral as is commonly used for laminated windshields or othersuitable interlayer material, and an inboard sheet of glass 12. Acoating 13 of the present invention is preferably placed on a surfacethat is not exposed, most preferably on the inboard side of the outboardglass sheet 10. The silver acts as the conductive layer and the zincstannate films serve to mask the reflectance of the silver. The coatingexhibits appropriate resistivity for use as a heated windshield when thesilver layer has a thickness of about 110 angstroms, for example.

An optional feature shown in FIG. 1, but omitted from FIG. 2 for thesake of clarity, is an opaque border 25 which may be ceramic enamelapplied to the flat glass surface by silk screening and fired on duringthe heating of the sheet for bending. The opaque border 25 serves toconceal attachment means and other elements when installed in a vehicle,and may also conceal the bus bars of the heating circuit.

Referring now to FIGS. 1 and 2, the electrical connections to the heatedwindshield embodiment shown are at the lower edge, at the center portionthereof. It should be understood that the connections could be at anyedge, and at an off-center location such as a corner region. A bottombus bar 15 and top bus bar 16 are in contact with the coating 13. Line14 indicates an edge of the coating 13 spaced from the sides and bottomedges of the transparency, leaving an uncoated margin along three sidesthereof. The uncoated marginal areas may be created by masking thoseareas during the coating process. Optionally the entire sheet could becoated and the coating subsequently deleted from those areas. Theuncoated marginal areas permit connections to be made to the upper busbar 16 without contact with the coating 13. As shown in FIG. 2, theconnections to the upper bus bar include two conductive strips 17 and 18extending in opposite directions along the bottom edge of thetransparency from the terminal area, and side strips 19 and 20 extendingalong opposite side portions which connect strips 17 and 18 to oppositeends of the upper bus bar 16. The bus bars and the conductive strips maybe made of the ceramic frit material containing silver well known in theart and which may be silk screened onto the glass surface (or onto theopaque border 25) and fused by heating. The conductivity of the bus barsand the conductive strips is chosen to be considerably greater than thatof the coating 13. Electrical lead 21 connects the lower bus bar to onepole of an electrical power source, and strips 17 and 18 leading to theupper bus bar may be wired in common to the opposite pole by means of ajumper wire 22 and lead 23. Referring now to FIG. 3, glass substrate 10is coated with a multilayer film 13 comprising a first transparent metaloxide layer 31; a first transparent, metal-containing primer layer 32; atransparent, infrared reflective metal layer 33; a second transparent,metal-containing primer layer 34; and a second transparent,antireflective metal oxide layer 35.

The present invention will be further understood from the description ofa specific example which follows. In the example, the zinc/tin alloyoxide film is referred to as zinc stannate although the film compositionneed not be precisely Zn₂ SnO₄.

EXAMPLE

A multiple layer film is deposited on a soda-lime silica glass substrateto produce a high transmittance, low emissivity coated product. Astationary cathode measuring 5 by 17 inches (12.7 by 43.2 centimeters)comprises a sputtering surface of zinc/tin alloy consisting of 52.4weight percent zinc and 47.6 percent tin. A soda-lime-silica glasssubstrate is placed in the coating chamber which is evaluated toestablish a pressure of 4 millitorr in an atmosphere of 50/50argon/oxygen. The cathode is sputtered in a magnetic field at a power of1.7 kilowatts while the glass is conveyed past the sputtering surface ata rate of 120 inches (3.0 meters) per minute. A film of zinc stannate isdeposited on the glass surface. Three passes produce a film thickness ofabout 300 Angstroms, resulting in a decrease in transmittance from 90percent for the glass substrate to 84 percent for the zinc stannatecoated glass substrate. A stationary cathode with a titanium target isfirst sputtered in an oxidizing atmosphere to deposit 30 Angstroms oftitanium oxide which reduces the transmittance to 82 percent, and isthen sputtered in an inert atmosphere to deposit 15 Angstroms oftitanium, reducing the transmittance to 72.5 percent. Next, a layer ofsilver is deposited over the titanium-containing primer layer bysputtering a silver cathode target in an atmosphere of argon gas at apressure of 4 millitorr. With the substrate passing under the silvercathode target at the same rate, silver is deposited to a film thicknessof about 100 Angstroms, further reducing the transmittance to 60percent. A second 15 Angstrom titanium primer layer is sputtered overthe silver layer, decreasing the transmittance to a low of 51.5 percent.Then the second antireflective layer of zinc stannate is deposited to athickness of 300 Angstroms, increasing the transmittance to 81 percent.

Optionally, a titanium cathode is sputtered at 10 kilowatts in anatmosphere comprising equal volumes of argon and oxygen at a pressure of3 millitorr to deposit a protective coating of titanium oxide about 15to 20 Angstroms thick. The protective coating of titanium oxide does notsignificantly affect the resistance and reflectance properties of themultiple-layer coating, and changes the transmittance no more than aboutone percent.

The improved durability of the coated article resulting from theimproved adhesion between the metal and metal oxide films as a result ofthe titanium oxide primer layers of the present invention is readilydemonstrated by a simple abrasion test consisting of wiping the coatedsurface with a damp cloth. A surface coated with zincstannate/silver/zinc stannate having no primer layers increases inreflectance from about 6 percent to about 18 percent after severalpasses of a damp cloth, indicating removal of both the top zinc stannateand the underlying silver films. In contrast, prolonged vigorous rubbingwith a damp cloth produces no visible change in a zinc stannate/titaniumoxide-titanium/silver/titanium/zinc stannate coated article comprisingthe primer layers of the present invention,

Preferred titanium oxide protective coatings have thicknesses in therange of about 10 to 50 Angstroms. Thicker titanium oxide coatings maybe used, limited only by the desired transmittance of the article. Witha titanium oxide protective coating about 20 Angstroms thick, thedurability of a multiple layer coating in accordance with this exampleis increased from 2 hours to 22 hours in a 21/2 percent salt solution atambient temperature, and from 5 hours to one week in the Clevelandhumidity test conducted with a Q-Panel Cleveland Condensation TesterModel QCT-ADO containing deionized water at 150° F. (about 66° C.).

The above example is offered to illustrate the present invention.Various modifications of the product and the process involvingtitanium/titanium oxide primers are included, such as varying theirrelative and absolute thickness, and may be carried out in such a primerat any location in a multilayer coating. The use of such a metal/metaloxide primer may also be practiced using zirconium. Other coatingcompositions are within the scope of the present invention, particularlyother antireflective metal oxides and infrared reflective metals such asgold or copper. Depending on the proportions of zinc and tin when azinc/tin oxide film is deposited, the coating may contain widely varyingamounts of zinc oxide and tin oxide in addition to zinc stannate. Thethicknesses of the various layers are limited primarily by the desiredoptical properties such as transmittance. Process parameters such aspressure and concentration of gases may be varied over a broad range.Protective coatings of other chemically resistant materials may bedeposited as either metal or oxides. The description of a windshield hasbeen set forth herein with reference to a particular embodiment for thesake providing the best mode of practicing the invention, but it shouldbe understood that variations and modifications known to those in theart may be employed without departing from the scope of the invention asdefined by the claims that follow.

I claim:
 1. A method for fabricating an electrically heatable coatedtransparency comprising the steps of:a. sputtering a metal cathodetarget comprising zinc and tin in a reactive atmosphere comprisingoxygen thereby depositing a first transparent antireflective zinc/tinoxide film on a surface of a transparent substrate; b. sputtering ametal-containing primer layer on said antireflective zinc/tin oxidefilm; c. sputtering an infrared reflective electroconductive metallicfilm over said metal-containing primary layer; d. sputtering ametal-containing primer layer over said infrared reflectiveelectroconductive metallic film; e. sputtering a second metal oxide filmover said metal-containing primer layer to produce an electroconductivemultilayer coating; and f. applying first and second bus bars to saidsubstrate in contact with said coating.
 2. A method according to claim1, wherein said substrate is glass.
 3. A method according to claim 1,further comprising the step of depositing a protective metal-containingovercoat deposited over said second metal oxide film.
 4. The methodaccording to claim 1, comprising the further step of subjecting themultiple-layer coated article to high temperature processing whereby thetransmittance of the coating increases.
 5. A method for fabricating anelectrically heatable coated laminated transparency comprising the stepsof:a. placing a transparent nonmetallic substrate in a sputteringchamber; b. sputtering zinc and tin in a reactive atmosphere comprisingoxygen to deposit a first transparent metal alloy oxide film on asurface of said substrate; c. sputtering titanium in an oxidizingatmosphere to deposit titanium oxide and then sputtering titanium in aninert atmosphere to deposit titanium to form a primer layer on saidoxide film; d. sputtering a silver cathode target in an inert atmosphereto deposit a transparent silver film on said primer layer; e. sputteringtitanium to deposit a second titanium-containing primer layer on saidsilver film; f. sputtering zinc and tin in a reactive atmospherecomprising oxygen to deposit a second metal alloy oxide film on saidsecond titanium-containing primer layer to produce an electroconductivemultilayer coating; g. applying first and second bus bars to saidsubstrate in contact with said coating; and h. laminating the coatedtransparent substrate with a second transparent substrate.
 6. The methodaccording to claim 5, wherein the transparent nonmetallic substrate isglass.
 7. The method according to claim 6, wherein the secondtransparent substrate is plastic.
 8. The method according to claim 6,further comprising the step of depositing a metal-containing protectivecoating over said second metal alloy oxide film.